Process for making aluminum-containing ferrites



United States Patent 3,418,241 PROCES FOR MAKING ALUMINUM- CONTAININGFERRITES Joseph H. Weis, Syracuse, N.Y., assignor to General ElectricCompany, a corporation of New York No Drawing. Filed Mar. 29, 1965, Ser.No. 443,695 12 Claims. (Cl. 252-6258) ABSTRACT OF THE DISCLOSURE Inaluminum-containing magnetic ferrites, the aluminum is more readilyintroduuced in the form of boehmite when making the ferrite.

This invention relates to magnetic ferrites. More particularly, itrelates to magnetic ferrites of the MgMnAlFe and YAlFe or YGdAlFe typeswhich are possessed of improved magnetic properties and to methods ofpreparing such ferrites.

Magnesium manganese iron ferrites and YGdAlFe ferrites have been foundto be useful in microwave devices at the X band (8,000 to 10,000megacycles) frequencies because of their low insertion losses, thesaturation magnetization of these ferrites ranging in the order of from2,000 to 2,400 gauss. It has been found that when aluminum is includedin ferrites of the above types to replace a part of the iron, it ispossible to reduce the saturation magnetization to as low as 300 gauss,thereby making such ferrites suitable for use at C band (6,000 to 8,000megacycles), at S band (3,000 to 6,000 megacycles), and lower than 3,000megacycles, depending upon the amount of aluminum present in thecomposition, the saturation magnetization being about inverselyproportional to the aluminum content.

In the preparation of magnesium manganese ferrites, manganese carbonateand magnesium carbonate are generally used becasue as carbonates theyare readily available, they are quite reactive and decompose at lessthan about 1000 C. Iron oxide (Fe O is also used. Its low cost andrelatively low melting point of 1565 C. facilitates preparation of theferrite. The resultant solid state material is MgMnFeO When aluminum isintroduced into the batch to lower the saturation magnetization for useat lower than X band microwave frequencies, processing problems areencountered because aluminum is generally introduced into the batch asA1 0 its most commonly available form. The reactivity of A1 0 is lowbecause of its refractory nature and high melting point of 2050 C. In anattempt to overcome the low reactivity of the A1 0 hydrated oxides andhydroxides of aluminum have been used as reactants. However, after thewater content is removed, the refractory oxide with its relatively lowreactivity tends to remain as such. The difficulty to introducingaluminum into magnetic ferrites has been recognized in the patent andother literature. It has been found that even with multiple calciningsto enhance the solid state reaction of the alumina, undesirable amountsof the A1 0 may remain as a distinct second phase ingredient unreactedwith the other components. This relative inertness of the A1 0 preventsor seriously limits the formation of magnesia manganese ferro aluminate,YAlFe and YGdAlFe, the sought for products.

From the above it will be quite apparent that there is a definite needfor magnetic ferrites of the above types containing aluminum in whichthe aluminum completely enters into the reaction and appears as anintegrated component of the resultant ferrite, and it is a principalobject of this invention to provide such magnetic ferrites and methodsof making such ferrites.

3,418,241 Patented Dec. 24, 1968 "ice Those features of the inventionwhich are believed to be novel are set forth with particularity in theclaims appended hereto. Further objects and advantages of the inventionwill, however, be appreciated from a consideration of the followingdescription.

It has been found that aluminum can be efliciently introduced into thestructure of ferrites, for example, magnesium manganese iron ferritesand YAlFe and YGdAlFe ferrites as an integral constituent by using asthe source of aluminum colloidal anisodiametric boehmite which iscapable of being readily incorporated into the ferrite structure. Thecolloidal anisodiametric boehmite (AlOOH), hereinafter referred to ascolloidal boehmite, is described in the literature, including Patent2,915,475, issued Dec. 1, 1959. Colloidal boehmite is an aluminamonohydrate having the boehmite crystal lattice in the form of particlesof colloidal dimensions which are anisodiametric or which do not haveequal diameters or axes. The form of the particles is rod-like andpreferably fibrous. The colloidal particles have an average length fromabout 10 to 1500 millimicrons at the extremes and have axial ratios ofat least 3:1. In the preferred case, the particles have lengths of 25 to1,000 millimicrons, the preferred fibrils being in the shape ofwell-formed little fibers or fibrils having at least one dimension inthe colloidal range or from 1 to millimicrons, with the fibril diametersbeing substantially uniform. The colloidal nature of the colloidalboehmite is indicated by its high specific surface area of 274 squaremeters per gram, its loose bulk density of 26 pounds per cubic foot andits absolute density of 2.28 grams per cc. Such colloidal boehmite issold under the name Baymal by Du Pont. Since boehmite (AlOOH) contains70.0 Weight percent of alumina after sintering, the amount of A1 0called for in any particular ferrite is replaced by about 1.4 times theamount of colloidal boehmite.

For example, it has been found that superior magnesium manganese ironaluminum ferrites are produced by reacting together from about one to 15mole percent colloidal boehmite calculated as A1 0 from about 45 to 55.0mole perecnt magnesium carbonate, from about one to 10.0 mole percentmanganese carbonate and from about 30 to 54.0 mole percent of Fe O Whenthe oxides are used, the proportions are about one to 15.0 mole percentcolloidal boehmite calculated as A1 0 21.5 to 26.3 mole percent MgO, 0.6to 6.6 mole percent Mn O and 30 to 54.0 mole percent Fe O The metalcontents are about 0.53 to 7.9 mole percent aluminum, 12.9 to 15.7 molepercent magnesium, 0.48 to 4.8 mole percent manganese and 21.0 to 37.7mole percent iron. Improved ferrites of the YAlFe and YGdAlFe types arealso provided as described below. In lieu of the carbonates, thecorrespondi g oxides can be used.

As opposed to the rigorous and expensive treatment in the form ofmultiple calcinings and relatively high temperatures required even forthe incomplete and generally unsatisfactory incorporation of A1 0 intosuch ferrite compositions, when colloidal boehmite is used as taughtherein, the aluminum is more readily incorporated into the ferritestructure. The coilloidal boehmite is mixed with the rest of the ferriteingredients as by dry mixing with a V- shaped twin cone or ribbon orspiral blade or other type of dry blender until such time as theindividual powders are indistinguishable when the product is rubbed on asmooth glass plate. The ferrite ingredients are readily mixed in the drystate according to the present invention, and a single calcining stepafter wet milling is generally sufiicient to promote complete solidstate reaction of the colloidal boehmite with the other materials.Sintering times and temperatures to form desired shapes are typical forthe magnesium manganese iron and other systems and can range from twohours to thirty-six hours of soak time at maximum temperature, the usualsintering temperature ranging fiom about 1250 C. to 1450 C.

Generally speaking, the raw powdered materials including the colloidalboehmite are mixed dry in a blender as described above and calcined atabout 700 C. to 900 C., and preferably at about 800 C., for from about 1to 8 hours to convert the magnesium carbonate and manganese carbonateconstituents to oxides if the carbonates of these constituents are usedas starting materials. If the corresponding oxides are used instead ofcarbonates, the dry blending and first calcining steps can be omitted.The dried, mixed powders derived from the carbonate process or the oxidestarting materials are wet milled in ball mills, on rolls, or inattritors or micronizers for from about 4 to 16 hours to produceintimately mixed powders whose grain size is less than about 10 micronsand preferably about two microns.This slurry is dried and the powdercalcined as in ceramic saggers or a rotary tube kiln for from about 1 to12 hours at from about 950 C. to about 1150 C., and preferably at about1050 C. for the MgMnAlFe ferrite and about 950 C. to 1250 C. when rareearths are added. The calcined powder is then wet milled normally forfrom about 4 to 24 hours with any desired binder such as polyvinylalcohol or other resinous material, natural or synthetic Waxes, oleates,stearates and the like and dried and pressed or molded into the desiredfinal shape. Other binders for such materials are well known to thoseskilled in the art. Finally, the pressed, bonded parts are sintered at1250 C. to 1450 C. after slowly burning ofI the organic content at theirvaporization temperatures. Such sintering can be done in air or oxygen.When a carbon dioxide atmosphere is used up to the end of the hightemperature soak period, annealing in oxygen is required beyond thisheating period for maximum fired density.

When copper in the form of copper oxide or carbonate is introduced intothe batch, the times and temperatures of treatment can be reducedbecause of the fiuxing effect of such copper compounds at hightemperatures.

Rare earths such as yttrium, gadolinium and the like can also beintroduced into batches in the ratio of 3 moles rare earth to each 5moles of iron oxide to impart desirable characteristics. Such desirablecharacteristics include high powder capabilities and low losses at lowerfrequency ranges such as S band. The aluminum replaces the iron oxide inamounts up to 1.25 moles A1 0 equivalent. These rare earth ferritecrystallize in the garnet system whereas MgMnAl ferrites crystallize inthe cubic system. Lesser or greater additions of rare earths result inpoorer properties.

Microscopic examination of the completed sintered ferrites reveals thecomplete absence of aluminum as a separate phase which indicates itscomplete solid state reaction with the other ingredients. This ascontrasted with the use of alumina which, unless subjected to intensefine milling or comminution and vigorous sintering remains as distinct Igrains in the ferrite matix because of its great hardness and extremerefractory nature. When colloidal boehmite is employed, the resultantferrite crystals are distinct with visible grain boundaries. Thecrystals range from about 1 to 40 microns in size depending upon thedegree of pulverization and heat treatment. Maximum pulverization andminimum heat treatment result in smaller grain dimensions. Low colloidalboehmite contents in the order of 1 mole percent, calculated as A1 0produce ferrites having Curie points over 300 C. and high saturationmagnetization of about 2100 gauss. When higher colloidal boehmitecontents of the order of 14 mole percent calculated as A1 0 are used,the Curie point is lowered to about 100 C. and the saturationmagnetization is reduced to approximately 850 gauss, permitting the useof the present materials in various microwave bands.

The following examples will illustrate the practice of the presentinvention, it being realized that they are to be taken as exemplary onlyof the general teaching of the present concept and its use of colloidalboehmite as a reactant ingredient for magnetic ferrite materials.

Example 1 There were dry blended in a mixer 49.6 mole percent MgCO 39.4mole percent Fe O 1.00 mole percent colloidal boehmite calculated as AlO and 10 mole percent MnCO the dry, mixed powder being calcined at atemperature of 800 C. for four hours to convert the carbonate to oxides.The calcined material was then wet milled in a ball mill using water forfour hours, the slurry then being heated to the dry state. The driedpowder was then calcined in a kiln for four hours at 1150 C. Next, thecalcined material was wet milled for 16 hours with 2 percent by Weightof polyvinyl alcohol. After such milling the material was dried intogranular free flowing grains and pressed into shape. The organic binderwas burned oif at a temperature of 200 C. and the pressed parts sinteredat a maximum temperature of 1350 C. for 36 hours, such sintering takingplace in oxygen. The saturation magnetization of the material was 2000gauss. The aluminum was completely reacted.

Example 2 Example 1 was repeated using as the starting materials 49.6mole percent MgCl 39.2 percent Fe O 6.7 mole percent colloidal boehmite,calculated as A1 0 and 4.5 mole percent MnCO .The final ferrite materialhad a saturation magnetization of 1600 gauss and the aluminum hadentered fully into the solid state reaction.

Example 3 Example 1 was repeated using as the starting materials molepercent MgCO 33.7.mole percent Fe O 13.3 mole percent colloidal boehmitecalculated as A1 0 and 3.0 mole percent MnCO The resultant ferritematerial had a saturation magnetization of 800 gauss.

As pointed out above, rare earth materials such as yttrium, gadoliniumand the like may be included in ferrite compositions to impart highelectrical power properties with low saturation magnetization for use atlower frequencies, such as S band. The following examples areillustrative of this aspect of the invention.

Example 4 Example 1 was repeated using as the starting materials 37.5mole percent Y O 13.75 mole percent colloidal boehmite calculated as A10 and 48.75 mole percent Fe O except that the first calcining wasomitted and the final heat reaction temperature ranged from about 1400C. to 1480 C., such heat reaction being carried out in an oxygenatmosphere for maximum density. The saturation magnetization of thisgarnet material was 500 gauss, making it suitable for use at S band.

Example 5 be used. Also, as pointed out above, the time and tem-'perature of treatment can be reduced by introducing copper compoundsinto the starting batch. Generallly speaking, the magnetic ferritesprepared according to the present invention have a fired bulk density ofabout 4.23 as compared with a like density of 4.07 when'aluminum oxideof 2.5 micron size is used, for 800 gauss magnesium ferrite.

There are provided, then, by the present invention magnetic ferriteswhich are particularly characterized by the thorough incorporation andreaction of the aluminum constituent. The materials prepared accordingto the present invention are characterized by improved magneticqualities such as lower losses and relatively higher permeabilities. Thehysteresis loop of the materials has a higher remanence promoted by therelatively low calcining temperature which makes the materialsparticularly useful in phase shifters. Their grain size is small anduniform. From the present teaching, it is made apparent to those skilledin the art how improved aluminum containing ferrites can be prepared.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The process of producing aluminum-containing magnetic ferrites whichcomprises introducing the aluminum in the form of colloidal boehmite.

2. The process of producing magnesium manganese aluminum iron ferritewhich comprises heat reacting together magnesium, manganese andiron-containing ingredients together with colloidal boehmite.

3. The process of producing yttrium aluminum iron ferrites whichcomprises heat reacting together yttrium and iron-containing ingredientsalong with colloidal boehmite.

4. The process of producing yttrium gadolinium aluminum iron ferriteswhich comprises heat reacting together yttrium, gadolinium andiron-containing ingredients along with colloidal boehmite.

5. The process of producing a magnetic ferrite of the magnesiummanganese aluminum iron types which comprises heat reacting togetherfrom about one mole percent to about 15 mole percent colloidal boehmitecalculated as A1 0 from about 45 mole percent to 55 mole percentmagnesium carbonate, from about one mole percent to mole percentmanganese carbonate, and from about 30 mole percent to 54 mole percentof F6203.

6. The process of producing a magnetic ferrite of the magnesiummanganese aluminum iron type which comprises heat reacting togetherabout 133 mole percent colloidal boehmite calculated as A1 0 about 50.0mole percent magnesium carbonate, about 3.0 mole ercent manganesecarbonate, and about 33.7 mole percent Fe O 7. The process of producinga magnetic ferrite of the magnesium manganese aluminum iron type whichcomprises heat reacting together about one mole percent colloidalboehmite calculated as A1 0 about 49.6 mole percent magnesium carbonate,about 10 mole percent manganese carbonate, and about 39.4 mole percentFe O 8. The process of producing a magnetic ferrite of the magnesiummanganese aluminum iron type which comprises heat reacting togetherabout 6.7 mole percent colloidal boehmite calculated as A1 0 about 49.6mole percent magnesium carbonate, about 4.5 mole percent manganesecarbonate, and about 39.2 mole percent Fe O 9. The process of producinga magnetic ferrite of the yttrium gadolinium aluminum iron type whichcomprises heat reacting together 24.7 mole percent Y O 12.6 mole percentGd O 7.7 mole percent colloidal boehmite calculated as A1 0 and 55.0mole percent Fe O 10. The process of producing a magnetic ferrite of theyttrium aluminum iron type which comprises heat reacting together about37.5 mole percent Y O 13.75 mole percent colloidal boehmite calculatedas A1 0 and 48.75 mole percent Fe O 11. The process of producingmagnetic ferrites which comprises heat reacting together ingredientscontaining about 0.53 to 7.9 mole percent aluminum, 12.9 to 15.7 molepercent magnesium, 0.48 to 4.8 mole percent manganese, and 21.0 to 37.7mole percent iron, the aluminum being introduced as colloidal boehmite.

12. The process of producing magnetic ferrites which comprises heatreacting together about 1 to 15 mole percent colloidal boehmitecalculated as A1 0 21.5 to 26.3 mole percent MgO, 0.6 to 6.6 molepercent Mn O and 30 to 54 mole percent Fe O References Cited UNITEDSTATES PATENTS 2,915,475 12/1959 Bugosh 23-141 2,981,903 4/1961 VanUitert 25262.5 3,006,856 10/1961 Calhoun et al. 252-625 3,108,88810/1963 Bugosh 23141 3,132,105 5/1964 Harrison et al. 252-62.5

TOBIAS E. LEVOW, Primaiy Examiner.

ROBERT D. EDMONDS, Assistant Examiner.

